Controlled Synthesis And Luminescent Properties Of Core/Shell Quantum Dots And Fabrication Of Light-emitting Devices

Bianry, multicomponent alloy and doped quantum dots (QDs) with core/shellstructures are typically synthesized with the hot-injection method, which is used tonot only control the core size but also change the shell thickness. These QDs haveexhibited size-and composition-tunable luminescent properties, highphotoluminesece quantum yield (PL QY), good stability, solution-processing andprinted films from ink, which make them promising in applications for light-emittingdevices (LEDs). Currently, QDs down-converters are being used for backlighting inLCD Vision and the quantum dot light-emitting diodes (QD-LEDs) with CdSe-basedQDs as emitter exhibit the maximum brightness of218800cd m-2, the maximumexternal quantum efficiency of18%and the lifetime recorded up to10000h.However, the performance of the QD-LEDs does not meet the requirement of thelarge-area display devices, therefore the device architectures and thecharge-transporting materials used for QD-LEDs need to be optimized to achievetradeoff between the luminance and efficiency, and a new recorded lifetime. Inaddition, the electroluminescent properties of cadmium-free CuInS2-based andMn-doped QDs with large stokes shifts have been fully on the top of potential rearch,which are interesting and promissing. Based on the above ideas，the original worksare given as follows: 1. We have synthesized Mn-doped ZnxCd1-xS QDs by growing a thin ZnS layeron the MnS nuclei and then growing a CdS layer with different thickness to vary thecomposition using nucleation-doping strategy. PL excitation spectra of the yellowemission (575nm) of Mn2+ions in Mn-doped ZnxCd1-xS QDs are tuned from425nmto475nm by growing the CdS layer. Combined morphology, crystal structure andsteady-state spectroscopy of the Mn-doped ZnxCd1-xS QDs indicate that the latticeenvironment of the host around Mn2+ions is ZnxCd1-xS alloy. Further, the PL ofMn-doped nanocrystals is composed of emissions of Mn2+ions and defect statesbased on temperature-dependent PL spectra and PL decay kinetics. We havefabricated inverted QD-LEDs using ZnO as the electron contacts and Mn-dopedZnxCd1-xS QDs with Zn/Cd ratio of0.64as the emitters, which exhibits a maximumbrightness of54cd m-2and a maximum current efficiency of0.056cd m-2, theelectroluminescence spectra are broader, exhibiting a large red shift relative to thePL spectra, which may orginate from Mn-Mn or defects related emission.2. We have synthesized Cu-Zn-In-S/ZnS core/shell QDs with hot-injectionmethod and characterized their morphology, crystal structure and luminescentproperties. The origin and mechanism of PL in Cu-Zn-In-S/ZnS core/shell QDs isdiscussed based on the routes of synthesis, temperature denpendent PL spectra andPL decay kinetics. Red and green Cu-Zn-In-S/ZnS core/shell QD down-convertersare used for fabrication of white LEDs, the CCT of the device increases at highcurrent, which is attributed to the thermal quenching of luminescence based on therecircled temperature-dependent spectra in the range of300K-400K-R300K and300K-480K-R300K. The inverted QD-LEDs with yellow QDs as emitter exhibit amaximum brightness of187cd m-2and a maximum current efficiency of0.22cd A-1.3. We have fabricated the inverted QD-LEDs with CdSe/CdS/ZnS core/shellQDs as the emitters with emission peak of624nm and full width at half maximum(FWHM) of24nm. The TiO2, TiO2/ZnO, and ZnO were utilized to deeply understandthe underlying mechanism of efficient inverted QD-LEDs with different metal-oxidematerials as the electron injection layer (EIL). The maximum luminance of the red QD-LEDs is2119,2908, and3861cd m-2for TiO2,TiO2∕ZnO, and ZnO-based devices,respectively. The peak current efficiency of the red QD-LEDs is5.0,3.9, and3.0cdA-1for TiO2,TiO2∕ZnO, and ZnO-based devices, respectively. In addition, the rise ofcurrent efficiency with increasing current density from0.1to20mA cm-2wasattributed to the negatively charged QDs.The introduction of the ZnO layer can leadto the formation of spin-coated uniform QD films and fabrication of high luminanceQD-LED. The TiO2layer improves the balance of charge injection due to its lowerelectron mobility relative to the ZnO layer. These results offer a practicable platformfor the realization of a trade-off between the luminance and efficiency in the invertedQD-LED with TiO2/ZnO composites as the electron contact layer.